(Reuters) - A lobbying group for major U.S. food manufacturers has violated campaign finance laws in its attempt to block a measure that would require labeling of genetically modified foods in Washington state, according to a lawsuit filed Wednesday by the state's attorney general.

State Attorney General Bob Ferguson alleges that the Grocery Manufacturers Association (GMA) illegally collected and spent more than $7 million while shielding the identity of its contributors.

GMA, which represents some of the largest food and beverage companies in the world, has been heavily lobbying against ballot initiative 522, set for a public vote in Washington state on November 5. The measure requires labels of goods containing ingredients made from genetically engineered crops as well as labeling of genetically engineered seeds and seed products sold in the state.

"Truly fair elections demand all sides follow the rules by disclosing who their donors are and how much they are spending to advocate their views," Ferguson said in a statement.

GMA is the largest donor to a "No on I-522" campaign, and the Attorney General's office said that the group set up a "Defense of Brands Strategic Account" within its organization and asked members to pay assessments that would be used to oppose the GMO labeling initiative. GMA then funded opposition efforts while shielding contributors' names from public disclosure, the attorney general alleges.

More than 300 companies are listed as members on GMA's website.

The attorney general's office said it would seek a temporary restraining order asking the court to order the GMA to immediately comply with state disclosure laws. The attorney general's office also said it would request civil penalties.

The group said in a statement that it was surprised by the lawsuit.

"GMA takes great care to understand and comply with all state election and campaign finance laws," the group said. "GMA will review its actions in Washington state and relevant statutes and continue to cooperate with state authorities to fully resolve the issue as promptly as possible."

The Yes on 522 campaign, which is pushing for labeling of GMO foods, said it was heartened by the lawsuit. The proponents of labeling are being heavily outspent in advertising and other campaigning efforts leading up to the election.

The latest campaign finance data reported to the state shows opponents raising more than $17 million and spending more than $13 million, compared to the proponents of labeling, who have raised about $5.5 million and spent about $5.4 million.

"It's clear that they broke the law," said Elizabeth Larter, spokeswoman for the Yes campaign. "They don't want to tell us who is funding the No on 522 campaign just like they don't want Washington consumers to know what is in their food."

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